1. Technical Field
The present invention relates in general to improved electrical connectors, and in particular to an improved surface treatment for electrical contacts in high reliability signal applications.
2. Description of the Related Art
Separable electronic connectors function as bridges or electromechanical interfaces for transferring low level, electronic signals from one electrical circuit to another. It is desirable to have minimal alteration of the amplitude or shape of the signals. To insure reliable operation, the mating surfaces of the connectors must have very low electrical resistances and be held tightly together. These objectives can be achieved with a physical design that incorporates a backbone made of spring metal containing a sandwich of metals at the mating area between the contacts.
The metallurgical properties of the mating surfaces at the area of contact is critical to long reliable performance of a connector. Good performance is typically achieved with a combination of coatings, platings, or claddings such as electroplated metals, each possessing unique properties. As shown in FIG. 1, the surface of a contact 11 typically has a substrate or base material 13 formed from less expensive spring materials like copper alloys (e.g., brasses, phosphor bronze, and beryllium copper), which exhibit good electrical properties. The first layer in the electroplating is usually composed of a hard underplate 15 such as nickel for good wear properties. Underplate 15 is approximately 50 microinches thick. The next layer is typically a soft noble metal overplate 17 such as gold, palladium, and palladium alloys (e.g., palladium/nickel). Noble metals are extensively used as the overplate. Overplate 17 is approximately 30 microinches thick.
Noble metals have excellent electrical conductivity with good chemical and environmental resistance at the point of contact. Since the most common failure mechanism for connectors is chemical attack with high resistance corrosion products at the contact interface, corrosion resistance to environmental conditions is critical to a long reliable life. Noble metals exhibit excellent corrosion resistance and, when plated in sufficient thicknesses, provide excellent protection. However, as thinner overplatings are becoming more popular, discontinuities such as breaks, pores, pits, etc., appear in the protective surface. Corrosion can penetrate through such discontinuities to the underlying base material and cause loss of performance. Moreover, noble metals are relatively expensive and they are time consuming to apply to the electrical contacting surfaces.
During manufacturing, the spring member or connector contacts are typically formed into long lines of breakapart individual contacts, much like a string of paper dolls with hands interconnected. The mating spots on each connector are located so that they may be dipped into electroplating baths and have the under and over plating applied in a continuous process. Overall connector cost is greatly influenced by the thickness and composition of each plating. Plating thickness is directly proportional to process time in a plating bath and affects throughput. Since noble metals or alloys are relatively expensive, thinner noble metal thicknesses reduce the cost of the connectors. The complete elimination of a plating layer would be a significant cost reduction. Although thinner noble metal platings are possible and would reduce connector costs, the connectors would have less corrosion protection.
Consequently, the connectors also would have less reliable and shorter product lifespans.
Many approaches have been attempted to improve the reliability of connectors in high reliability applications. In particular, attempts have been made to overcome reduced reliability with less expensive or thin platings. Coating connector contacts with oils and greases reduces mating friction, improves wear life, and provides some environmental protection. Very specialized oils have been developed for the connector industry, both for signal and power applications. Although wear at the contact mating surfaces can be improved with the application of lubricants, environmental protections is only moderate at best. Thus, an alternative solution for high reliability electrical contact surfaces that provides excellent corrosion resistance at a lower cost would be highly desirable.
One embodiment of an electrical contact has a copper alloy substrate and a hard underlayer plating such as nickel. The underlayer is coated with a thin, liquid barrier film coating. The substance that forms the coating is one of the proprietary materials that are described in military specifications MIL-C-81309E, and MIL-L-87177A, Amendment 1. The military specifications generally describe classes of ultra-thin film, water-displacing, corrosion preventive compounds that may be applied by dipping, brushing, or from gas-pressurized containers. The mating surfaces of both mating contacts (i.e., male and female) are provided with the coating. The coatings provide excellent corrosion protection for both contacts despite physical contact therebetween.
The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the preferred embodiment of the present invention, taken in conjunction with the appended claims and the accompanying drawings.